The development of an instrument for the direct measurement of lung dose resulting from exposure to indoor radon is proposed. The objective is a technology which makes possible much better estimates of the risk to persons exposed to indoor radon than has been possible heretofore. Our methods for achieving this are based on the use of collectors to simulate the collection of radon progeny within the human respiratory tract and an alpha particle spectrometer to detect and measure the energy of alpha particles that would be produced by these radon progeny. Inhaled radon progeny are the dominant sources of background radiation that deliver significant doses to the tissues or cells of the respiratory tract. Phase I will result in the construction, testing, and calibration of a laboratory working model of the collector/detector system and the demonstration that an inexpensive solid state detector and electronics can be reliably utilized. Phase II will configure the technology into an inexpensive system for real time measurement of lung dose. Phase III involves commercialization of this patented technology through fabrication and sales of various stand-alone instruments based on the technology.